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Pathophysiology of Gastrin and Secretin

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Pathophysiology of Gastrin and Secretin J Clin Pathol: first published as 10.1136/jcp.s1-8.1.17 on 1 January 1978. Downloaded from J. clin. Path., 33, Suppl. (Ass. Clin. Path.), 8, 17-25 Pathophysiology of gastrin and secretin KEITH D. BUCHANAN AND JOY E. S. ARDILL From the Department of Medicine, Queen's University of Belfast, Institute of Clinical Science, Grosvenor Road, Belfast BT12 6BJ This paper will attempt to define the pathological THE NATURE OF GASTRIN roles of gastrin and secretin. It will deal predomin- Like many other peptide hormones gastrin exists in antly with gastrin which now has an accepted role in multiple molecular forms. The reason why peptide some diseases, whereas the pathological role of hormones exist in multiple forms is still uncertain secretin has still to emerge. Basic physiology and although in many instances, as in the case of pro- chemistry will only be described where these are insulin and insulin, the relationship is likely to be considered to be relevant to the interpretation of the that ofa precursor and active hormone. Preparations roles of these hormones in disease. of gastrin from gastrin-producing tumours, from normal tissues, and from circulating plasma have Gastrin been characterised. It is fortunate that in several conditions blood levels of gastrin are very high, The history of gastrin began in 1905 when Edkins making investigation of circulating forms easier than copyright. observed than an extract of gastric antral mucosa in the case of some other peptide hormones. Al- proved to have extraordinary potency in stimulating though considerable progress in the characterisation gastric acid secretion. Approximately 60 years later of tissue gastrins has been achieved it is still not gastrin was isolated and characterised by Gregory certain how closely these preparations resemble the and Tracy (1964). It is composed of a single peptide circulating species. chain of 17 amino-acids (G17) with a molecular Gregory and Tracy in 1975 isolated from both weight of 2100. Two forms of gastrin, I and II, were gastrinomas and hog antral mucosa two peptides recognised initially, the only difference being that in containing 34 amino-acids. This gastrin has been http://jcp.bmj.com/ II the tyrosyl residue in position 12 is sulphated. referred to as G34 (big gastrin) in contrast with the The synthesis of human gastrin was achieved by heptadecapeptide G17 (small gastrin). Like G17 the Morley in 1968. two G34s differ only in the presence or absence of a The predominant biological activity of gastrin is sulphate on the tyrosyl residue. A 'mini-gastrin' has that of increasing gastric acid production. Several been isolated in small amounts from gastrinomas other roles have been assigned to it including trophic (Gregory and Tracy, 1975) and has been shown to effects on the stomach, effects on the exocrine have the amino-acid composition of the COOH on September 29, 2021 by guest. Protected pancreas, and on the enteroinsular axis, but they do terminal tetradecapeptide (G14). Another variant not appear to play a role in pathological processes with the amino-acid composition of the 1-13 and will not be discussed further. fragment of G17 has been isolated from antral Gastrin is an antral hormone secreted by 'G cells' mucosa (Gregory, 1974). in the mid zone of the glands. However, smaller Components in the plasma which correspond amounts of gastrin are present in the duodenum and with these different gastrins have been identified in jejunum and, in some species, there may be small some instances (Rehfeld and Stadil, 1973). Com- amounts in the pancreas. Recently a substance which ponent m ofplasma corresponds to G17, component reacts with antigastrin antiserum has been detected II corresponds to G34, but component I of plasma, in the central nervous system (Vanderhaeghen et al., which elutes from Sephadex G50 between the void 1975). Immunochemical evidence so far suggests volume and G34, appears to have no tissue counter- that the 'brain gastrin' consists of COOH terminal part. An additional component in plasma, big big fragments of cholecystokinin (Dockray, 1976). The gastrin (BBG), which elutes from Sephadex G50 in reason for the existence of gastrin-like peptides in the void volume, has been noted by Yalow and the central nervous system is not known and so far Berson (1972). It constitutes the major fraction of they have not been shown to play a role in any the immunoreactive gastrins extracted from fasting pathological process in man. plasma but does not increase with feeding; it seems 17 J Clin Pathol: first published as 10.1136/jcp.s1-8.1.17 on 1 January 1978. Downloaded from 18 Keith D. Buchanan and Joy E. S. Ardill likely to be an artefact which occurs on gel filtration by the groups of Gregory in Liverpool and Rehfeld of whole plasma as a result of non-specific binding in Copenhagen. In addition radioimmunoassay has ofgastrin peptides to plasma proteins, a phenomenon made it possible to measure many samples simul- observed frequently with other peptide hormones. taneously. However, there may be as yet unknown The amino-acid sequences of some of the gastrins species of gastrin which cannot be detected by are shown in Fig. 1 and the relationships between available antibodies, for example in large molecular tissue gastrins and plasma gastrins are illustrated in species of gastrin the areas recognised by the anti- Table 1. bodies may be hidden by other unknown and non- Plasma gastrin levels in fasting normal subjects do immunoreactive areas of the molecule. In addition not correlate with basal acid secretion rates. This is tiny fragments of gastrin could be present in the perhaps not surprising as, in the fasting state, BBG, circulation but unrecognised by any existing gastrin which is probably an artefact, constitutes the major antibody. part of the immunoreactive gastrin in plasma. This lack ofcorrelation may be partly because of different METHODS OF INVESTIGATION proportions of G34 and G17, as G17 has a shorter To assess the role of gastrin in health and disease half life than G34 in the circulation, though it is five the following techniques are required: radioimmuno- times more potent than G34 in stimulating acid assay, immunohistochemistry, electron microscopy secretion (Walsh et al., 1975). of G cells, and physical and chemical methods for Finally, it should be stated that the ability to the characterisation of gastrin, including gel detect multiple forms of a peptide hormone depends filtration and bioassay or assays using receptor site on the methods available. The considerable advances binding. Of these, radioimmunoassay has made the in the chemistry of gastrin have only been possible greatest contribution in recent years to the under- because of the use of multiple chemical techniques standing of gastrin pathophysiology. Gastrin can be assayed in plasma, serum, or tissue extracts. It iscopyright. relatively stable in blood, does not adhere to glass- Tissue gastrin Equivalent form ware, and can be assayed in plasma without pre- in plasma liminary extraction. The gastrin radioimmunoassay in use in our Big big gastrin Component I department was described by Ardill (1973). The Big gastrin (G34, non-sulphated and sulphated) Component II antibody 0098 was raised in rabbits against synthetic Gastrin (little gastrin) (G17) I non-sulphated Component III human gastrin I (Imperial Chemical Industries), http://jcp.bmj.com/ II sulphated containing amino-acids 2 to 17. The only other known Mini gastrin (a) G14 (residues 4 to 17 of G17) Component IV gastrointestinal hormone with which this assay (b) G13 (residues 1 to 13 of G17) cross reacts is cholecystokinin, 1:10 000 by weight. Table 1 Gastrins in human plasma and tissue The antibody also recognises gastrin G34 in equi- molar amounts. The gastrin is iodinated by the The plasma components still await full chemical identification, and only the probable tissue gastrin equivalent of the component is given. chloramine-T method and the product purified by There is no tissue equivalent for 'big big gastrin' or component I. gel filtration. Equivalent amounts of plasma on September 29, 2021 by guest. Protected 1 2 3 4 57 6 7 19 1 11 12 13 14 1517 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 (a) Pyr - Leu-Gly -Pro-Gln-Gly- His- Pro- Ser- Leu-Vcl-AIa-Asp-Pro -Ser-Lys- Lys-Gln-Gly-Pro-Trp-Leu- Glu-Glu- Glu-Glu-Glu-Ala-Tyr-Gly -Trp-Met-Asp - Phe l l R NH2 1 2 3 4 5 6 7 8 9 10 1213 14 15 16 17 (b) Pyr-Gly-Pro-Trp -Leu -Glu-Glu- Gtu -Glu-Glu-Ata-Tyr-GIy-Trp -Met-Asp -Phe l I R NH2 1 2 3 4 5 6 7 8 9 10 11 12 B 14 (c) Trp- Leu-Glu-Glu-Glu-Glu-Ghu-Alo-Tyr-Gly1l -Trp-Met-Asp - Phe R NH2 Fig. 1 Amino-acid sequences ofhuman gastrins. (a) Human big gastrin (G34) MW 3839. (b) Human heptadecapeptide, little gastrin (G17, R = H or SOaH residues, 18-34 ofG34) MW 2098. (c) Huran minigastrin (G14, residues 21-34 of G34, 4-17 of G17). Pyr, pyroglutamyl; gastrin I, R = H; gastrin II, R = SOaH. J Clin Pathol: first published as 10.1136/jcp.s1-8.1.17 on 1 January 1978. Downloaded from Pathophysiology ofgastrin and secrettin 19 adsorbed with charcoal (gastrin free) is added to blood gastrin estimation in the diagnosis ofduodenal the standards. The assay has a sensitivity of 5 ng/l ulcer (DU). Between June 1975 and February 1977, but this sensitivity can be improved by a variety of which was before the introduction of H2-receptor manipulations including the use of disequilibrium blocking drugs, gastrin assays were performed in conditions, reducing the concentrations of labelled 223 patients diagnosed provisionally as DU or hormone and antibody, and increasing the incubation Zollinger-Ellison syndrome (ZES).
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